Silicon nitride has been an important material in various semiconductor applications. For instance, silicon nitride has been used as a mask against oxygen diffusion during a local oxidation (LOCOS) process; as a passivation layer for its superior barrier property to contaminants; as a gate dielectric layer in memory devices; and as an interlevel dielectric layer in an oxide-nitride-oxide (ONO) stacked-gate structure. Silicon nitride also has superior barrier properties against metal ions and moisture.
Silicon nitride has been widely used as a passivation layer for protecting a semiconductor component. Silicon nitride can be formed by either a LPCVD or PECVD technique. The LPCVD technique, where dichlorosilane is used as the reactant gas, can be carried out in a hot-wall LPCVD system, such as in a vertical furnace. The chemical reaction can be described as follows:
3SiH.sub.2 Cl+10NH.sub.3.fwdarw.Si.sub.3 N.sub.4 +6NH.sub.4 Cl+6H.sub.2
The hot-wall LPCVD system is normally carried out at a temperature between about 750.degree..about.800.degree. C. and the chamber pressure is kept at several hundred m Torr. A layer of stoichiometric silicon nitride can thus be deposited on a wafer surface. A typical deposition equipment utilizing a vertical furnace is shown in FIG. 1.
During a vertical furnace silicon nitride deposition process, as described by the above mechanism for the chemical reaction, a reaction by-product such as ammonium chloride (NH.sub.4 Cl) in the form of a fine powder can easily deposit on any cold surface in the furnace or in the ducting system for the furnace. The ammonium chloride powder must be captured by a cold trap such that it does not form on the inner walls of the ducting system or in the furnace presenting a serious contamination source. For instance, fine powder in the ducts may be syphoned back into the furnace during a deposition process if the pressure in the furnace is not carefully controlled. The capture efficiency of the cold trap for the ammonium chloride fine powder is therefore an important factor in the successful deposition of silicon nitride films in a furnace technique.
As shown in FIG. 1, a vertical furnace unit 12 is the heart of a silicon nitride deposition system 10. During the deposition of a silicon nitride film on a plurality of wafers positioned in the vertical furnace, the furnace exhaust gas 14 which contains unreacted reactant gases such as dichlorsilane, ammonium and reaction byproduct ammonium chloride powder is sent through a cold trap 20 before it enters into a gas treatment unit 18 and be released into a factory exhaust system 22. The capture of substantially all the ammonium chloride fine powder in a cold trap 20 is therefore an important step in a successful exhaust gas treatment process for depositing silicon nitride.
A cross-sectional view of a conventional cold trap 20 complete with an inlet 24 and an outlet 26 is shown in FIG. 2. The cold trap 20 is normally constructed of a curvilinear housing 28 which supports a cooling plate 30 therein. The cooling plate 30 has a flat platen structure with a cavity contained therein for allowing a cooling medium to pass therethrough. A suitable cooling medium can be chilled water, i.e., chilled deionized water or city water at a temperature of about 15.degree. C. The cooling plate 30 is normally constructed in a rectangular shape, i.e., having a dimension of about 4 inch.times.6 inch and is equipped with a plurality of cooling fins 32 extending from a front surface 36 of the cooling plate 30. The cooling fins 32 are provided to facilitate the evaporation of heat absorbed by the cooling medium in the cavity and furthermore, to provide a cold surface for the deposition of ammonium chloride powder. The cooling plate 30 is further provided with a cooling medium inlet and a cooling medium outlet (both not shown) for the input and output of the cooling medium into and from the cavity.
In the configuration of the cooling plate 30 shown in FIG. 2, the cooling plate and the spherical housing 28 must be frequently cleaned, i.e., by a preventive maintenance cleaning procedure in about every two weeks. Fine particles 38 of ammonium chloride tend to clog the cooling fins 32 and thus blocking the inlet 24 for the exhaust gas. The requirement for frequent cleaning of the cold trap 20 therefore presents a problem in the silicon nitride furnace system in causing down time which reduces the fabrication yield.
It is therefore an object of the present invention to provide a cold trap that can be efficiently used in a semiconductor fabrication process for collecting unwanted particles that does not have the drawbacks or shortcomings of the conventional cold traps.
It is another object of the present invention to provide a cold trap that can be used effectively in a semiconductor material deposition system such that the cleaning frequence required for the cold trap can be reduced.
It is a further object of the present invention to provide a cold trap for use in a semiconductor fabrication process which does not need frequent cleaning.
It is another further object of the present invention to provide a cold trap for use in a semiconductor fabrication process that is equipped with a curvilinear cooling plate.
It is still another object of the present invention to provide a cold trap for use in a semiconductor film deposition system that is equipped with a convex cooling plate equipped with cooling fins extending from the convex surface.
It is yet another object of the present invention to provide a cold trap for use in a vertical furnace silicon nitride film deposition system which is equipped with a curvilinear cooling plate.
It is still another further object of the present invention to provide a cold trap for use in a silicon nitride furnace deposition process that is equipped with a convex cooling plate and cooling fins in the convex surface.
It is yet another further object of the present invention to provide a cold trap for use in a vertical furnace for depositing silicon nitride films wherein the trap has greatly improved efficiency for trapping ammonium chloride fine powder.